The discovery that RNA can catalyze reactions has changed our view of the role of RNA in biological systems. There are presently a large number of systems for which RNA has been shown to be the biological catalyst, and the hammerhead RNA cleaving domain is one of the best studied systems. Although there is a great deal of information on the biochemical and kinetic properties of the hammerhead domain, there is presently no direct structural data on this system. Thus we will use nuclear magnetic resonance (NMR) spectroscopy to generate structural data on the hammerhead self-cleaving RNA domain and also on a related less well-characterized system, the hairpin self-cleaving RNA domain. We will primarily concentrate on determining the tertiary structure, the structure of the active site, and structure-activity relations in both systems. The hammerhead and hairpin ribozymes have site-specific RNA endonuclease activities and have been shown in vitro to efficiently and catalytically cleave specific sites in target RNAS. These results suggest important in vivo applications where one could design a ribozyme to specifically cleave a target mRNA or the genome of an RNA virus such as HIV. The direct structural information from our NMR studies combined with the kinetic, mechanistic and biochemical properties of these ribozymes will aid in the design of optimal RNA cleavage reagents.